1. Field of the Invention
The invention relates to a sensor arrangement for flow injection analysis, comprising an enzyme reactor and a chemical/physical sensor. Enzyme reactor is here understood to mean all immobilised biological components, such as enzymes, antibodies or the like, which can ensure a particular substrate specificity.
2. Brief Description of Related Art
Flow injection analysis (FIA) is a wet-chemical analysis procedure which in recent years has developed to become a valuable and frequently used method. In flow injection analysis, the sample to be analysed is injected into a stream of an appropriate liquid (carrier liquid) and is conveyed together with this liquid to a detection system. In the detection system the passage of the sample is established and the analyte content of the sample is quantitatively determined in an evaluation device from the output signal. Although flow injection analysis is not a continuous analysis procedure, the repetition rate of the individual measurements is nevertheless generally so high that in many cases of application the procedure can be regarded as being quasi-continuous.
Flow injection analysis thus makes it possible to transfer conventional analytical laboratory techniques to a continuous flow process. It is possible to carry out fully automatic pipetting, diluting and mixing processes as well as chemical reactions or measurements by means of electrodes or optodes.
Suitable detectors are any devices which convert the chemical detection reaction quantitatively into an electrical signal. Examples of detectors are chemical/ physical sensors and transducers such as photometers, fluorometers, refractometers, luminescence detectors, turbidimeters, pH- and ion-sensitive electrodes, voltametric and amperometric detectors, conductimeters, thermistors, semiconductor structures (FETs) etc.
One precondition for an error-free analysis is that there should be no air bubbles present in the liquid stream, since this frequently results in completely false analysis results. The injection volume is reduced, leading to mixing and reaction errors and to measurement errors in the detection system. Measures for preventing or eliminating air bubbles in flow injection systems consist in degasing the liquids used, which is relatively expensive and not without risk. A recent proposal for counteracting air bubbles still present in the system involves the use of air bubble traps which catch the air bubbles present in the flow system and convey them onwards.
Nor is flow injection analysis restricted to wet-chemical determination. By using enzymes in dissolved or immobilised form, immobilised antigens/antibodies, organelles and microorganisms, the range of application of flow injection analysis can be greatly widened.
The use of enzymes in flow injection analysis permits a specific determination of analytes in complex media too. The enzymes are usually immobilised, i.e. used in an enzyme reactor which permits the determination of enzymatic conversions by the analyte, for example the consumption of educts such as, for example, oxygen and for formation of products such as, for example, carbon dioxide or of acids.
For achieving accurate analysis results, the biological components have hitherto been applied directly to the chemical/physical sensor, and the chemical/physical sensor (transducer) has been combined in one component part. The arrangement is in this case such that the analytes must diffuse through the enzyme layer to the chemical/physical sensor. The considerable spatial proximity prevents the concentration of the analyte, to be determined by the chemical/physical sensor, from rapidly decreasing as a result of dispersion or gas exchange with the environment. However, in order not to alter the response behaviour and the linear measuring range in an undesirable manner, due to the given geometry of the component part only a limited amount of enzyme can be used, i.e. an enzyme layer of specific thickness. As a result of this the sensor arrangement is very sensitive to interference.
Another sensor arrangement for flow injection analysis has been developed in which the biological component is separated from the chemical/physical sensor. This construction is particularly advantageous for biosensors with enzymes of low activity and/or stability. In contrast to the layer arrangement of biological component and transducer, the spatial separation allows a greater amount of enzyme to be used. The enzyme excess thus obtained results in a decrease in the sensitivity of the system in relation to interference ions (e.g. heavy metal ions) and fluctuating pH values. However, one disadvantage of the separation of the biological component from the chemical/physical sensor is that the gases to be determined can escape in the sensor arrangement, so that these gases may be detected only incompletely by the downstream chemical/physical sensor. This occurs in particular at low flow rates.